Dual monitor, game shows FPS drop of at or over 50%?!

So i just got my long wanted second monitor this afternoon. But after i hooked it all up, i noticed that whenever i tried to play a game on my main monitor and do different things with the other monitor, i starting dropping in framerate. And not just a bit, quite drastically. Pretty much starting at 50% and worse.

Now at first, a friend of mine suggested i would set both monitors at the same refresh rate. Since the main monitor is a 120Hz LCD and the offmonitor is a 60Hz LCD. So I did. It helped, but nowhere near acceptable levels. It's still far to big of a loss to be normal.

The weird thing about it is.. sometimes it's fine.. and sometimes it's not.

The only permanent solution i managed to find myself was putting my game at fullschreen, instead of fullschreen (windowed)..
But that is kind of not what i want. Since it minimizes the game when i use the alternative monitor.

I'm running an EVGA GTX 670 FTW edition, one monitor (120Hz) being connected via DVI-D. And the alternative (60Hz) being connected via HDMI.
Both directly into the GPU ofcourse.

Can anyone solve this massive framerate lag problem for me? I would highly appreciate it.

Need more information? Just ask, I've build the machine myself so i know exactly what it is made of.

when adding a second monitor you having to "render" twice as much screen information, so there will be a FPS drop.
What processor are you using ? and when you say a drop in FPS, what are you before and after FPS results ?

when adding a second monitor you having to "render" twice as much screen information, so there will be a FPS drop.
What processor are you using ? and when you say a drop in FPS, what are you before and after FPS results ?

Yeah ofcourse. But not a 50%+ FPS drop. It might be twice the pixels but it's not twice the power needed since the alternative doesn't run any game.

I use a home-overclocked i5-3570k @ 4400MHz.

So far, i've only been playing SWTOR.. however.. after trying a few other games.. it seems that it is indeed isolated just to that game.

Try putting the game in windowed mode i have had this in a few games in the past and normally windowed mode works as simple as it sounds

I can't believe that actually worked..

One more thing now though, pressing certain button combination sometimes plays the windows error sounds (ding) for example if i press Alt+F sometimes it does that. Is there anything against that, or will i just have to cope with it?

when adding a second monitor you having to "render" twice as much screen information, so there will be a FPS drop.
What processor are you using ? and when you say a drop in FPS, what are you before and after FPS results ?